CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional application No. 60/004,814, filed Oct. 5, 1995.
1. Field of the Invention
This patent relates to a magnetic glassy metal alloy for use in an electronic article surveillance system; and more particularly to a method for heat-treating the alloy to improve the electromagnetic performance thereof.
2. Description of the Prior Art
Article surveillance systems are commercially available in the industrial and retail markets to aid in identification of people or objects. An essential component of such systems is a sensing element or "marker", that is attached to the object to be detected. Other components of the system include a transmitter of an interrogating signal and a receiver of the signal response from the marker. The information contained in the response signal is then processed for actions appropriate to the application, such as denial of access, triggering of an alarm, sorting of articles, and the like.
Several different types of marker-detection systems have been disclosed and are in use. Each marker has to be compatible with a pertinent detection system. Taking advantages of electromagnetic phenomena, three types of marker-detection systems are currently in use: they are based on radio-frequency resonance (hereinafter the "rf marker") utilizing a simple antenna-capacitor-diode circuit, detection of higher harmonics response signal from a magnetically high permeable marker (hereinafter "harmonic marker") and detection of response signal from a magneto-mechanically excited marker (hereinafter "magnetomechanical resonator"). One of the most troublesome problems associated with the first two markers is the low detection reliability thereof. This problem is primarily due to the inherent broadband nature of a simple resonant circuit for an rf marker, and the rapid decay of the response signal generated by a harmonic marker as the distance between the marker and the signal receiver increases. A magneto-mechanical resonator is a semi-active element and, as such, offers a combination of high detection sensitivity, high operating reliability and low operating costs. Examples of such systems are disclosed in U.S. Pat. Nos. 4,510,489 and 4,510,490 (hereinafter the "489" and "490" patents).
The marker in a mechanical resonator based system is a strip, or a plurality of strips, having a known length. Each strip is composed of a magnetically soft ferromagnetic material and is packaged with a magnetically harder ferromagnetic material having a higher coercivity. The harder ferromagnetic material provides a biasing field that establishes peak magneto-mechanical coupling. The soft ferromagnetic marker material is preferably a metallic glassy alloy ribbon, since the efficiency of magneto-mechanical coupling in glassy alloys is very high. The mechanical resonance frequency of the marker material is dictated essentially by the length of the glassy alloy ribbon and the biasing field strength. When an interrogating signal tuned to the resonance frequency is encountered, the marker material responds with a large signal field which is detected by the receiver. This large signal is attributable in part to an enhancement of the marker's magnetic permeability at the resonance frequency. Various marker configurations and systems for the interrogation and detection that utilize the aforementioned principle have been disclosed in the '489 and '490 patents.
In one particularly useful system, the marker material is excited into oscillations by pulses, or bursts, of signal at its resonance frequency generated by the transmitter. When the exciting pulse is over, the marker material undergoes damped oscillations at its resonance frequency. That is to say, the marker material rings down following the termination of the exciting pulse. A receiver listens to the response signal generated during this ring down period. With this arrangement, the surveillance system is relatively immune to interference from various radiated or power line conducted sources, and the potential for false alarms is reduced.
A broad range of alloys have been disclosed in the '489 and '490 patents as being suitable for marker material, for a magneto -mechanical detection system. Other metallic glass alloys bearing high permeability are disclosed in U.S. Pat. No. 4,152,144.
Although many glassy alloys have been disclosed as being suitable as markers for article surveillance systems of the magneto-mechanical variety, commercial systems of this type typically utilize markers composed of as-cast, iron-nickel base glassy alloys. It would be particularly desirable if the size and weight of these markers could be decreased and the signal amplitude thereof increased. However, attempts to accomplish such improvements have been thus far unsuccessful. There remains a need in the art for an improved mechanically resonant marker which is small and light, and resists degradation when subjected to mechanical deformation.